1. Technical Field
This disclosure generally relates to a mechanical gripper adapted for use in an autonomously functioning device, such as a robot; and more particularly, to a mechanical gripper comprising a plurality of fingers that utilizes active material activation to facilitate grasping a plurality of differing objects (e.g., workpiece), and effect variable mechanical impedance.
2. Background Art
Fixed impedance mechanical grippers have long been used in devices to autonomously grasp objects. For example, jaws comprising rigid integral members and hinge joints that are articulated by conventional actuators, such as electric motors, solenoids and pneumatic cylinders, are often used in assembly robots to grasp and manipulate workpieces.
Variable impedance passive compliant grippers, including both stiff and flexible elements, have more recently been developed to avoid damaging workpieces and surrounding environments. The stiff elements promote precision, while the flexible elements enable passive compliance. This duality makes the grasp more robust to small variations of the position or configuration of the workpiece, or to the workpiece environment. Concernedly, it is appreciated that passive compliance limits the performance (payload capacity, speed of motion, etc) of the gripper.
The type of grip employed by a mechanical gripper plays a major role in determining the magnitude of force required for a stable grasp, and generally include two types, “friction” or a precision grasp, and “encompassing or power grasp.” Friction grippers rely completely on the frictional force between the object and the individual grasp fingers, which is directly proportional to the gripping force, wherein the constant of proportionality (μ—the coefficient of static friction) is typically within the range 0.2 to 0.25. When fragile or delicate workpieces need to be handled by frictional robotic grippers, complex feedback systems are required to ensure that the work piece is not damaged during the grasping process or during transport. Concernedly, however, feedback control typically requires precise motion sensing and correction, complexity (including the injection of extra energy to correct torque/joint force), and additional hardware and software to implement the control strategy, which present barriers to entry and add significant costs to production. For example, it is appreciated that feedback control often requires a large plurality of small parts that increases repair and maintenance costs. Moreover, where the location, orientation, shape, or stiffness/strength of the workpiece to be grasped is not known with sufficient certainty, and a friction gripper is employed, the gripper must typically be oversized to account for the worst case scenario. The extra mass presented thereby and force necessary for actuation, complicates control and results in inefficiencies and waste.
Encompassing grippers cradle the part, add stability to the grasp, and as a result, typically require a smaller gripping force by the individual grasp fingers (e.g. a factor of 1 to 4) than do friction grippers. This type of gripper, however, also presents concerns in the art, including, for example, one-size-fits-all configurations that, in some cases, are not able to grasp objects and workpieces of differing dimensions and shapes. More specifically, the individual fingers of encompassing grippers, though inwardly and outwardly translatable (e.g., pivotal, etc.), typically present relatively fixed or limitedly adjustable configurations. It is appreciated that conventional friction and encompassing grippers are generally not interchangeable.